This invention relates to aqueous compositions, e.g. coating compositions such as latex paints and to the process for preparing dispersions of pigments and polymeric resins in aqueous mediums with a dispersing agent. The dispersing agent consists of mixed esters of starch characterized as having pendant carboxylic acid groups and an average degree of substitution ranging from 0.5 to 3.0 wherein at least about 10 mole percent of the pendant carboxyl groups are neutralized. These mixed esters of starch or dispersants are derived from the low molecular weight hydrolyzed starch molecules having a plurality of anhydroglucose units and a combination of acylating agents consisting of at least one anhydride of a polycarboxylic acid and at least one anhydride of a monocarboxylic acid and/or the acyl halide of said monocarboxylic acid.
The incentive for developing dispersants derived from starch is the uncertainty over the availability of similar materials derived from petrochemicals. A way of avoiding the problems encountered due to shortage of materials prepared from petrochemicals is to diversify the raw materials from which the compounds are derived by relying on agricultural products. These products are renewed on an annual basis and the volume, if necessary, can be expanded in accordance with the demands by merely increasing the acreage. Starch, for example, which is derived from corn, potatoes, rice or the like is presently being produced at rates exceeding ten billion pounds per year and is being used for a variety of industrial purposes. Starch in most instances, however, must be chemically modified, e.g. esterified before products can be obtained with the necessary characteristics and particularly for use in aqueous systems. Low molecular weight starches with high degrees of substitution are not commercially available and for the most part only the high molecular weight starches with low degrees of substitution are being used in most commercial applications.
The standard grade of a high molecular weight starch consists of approximately 80% amylopectin with average molecular weights ranging from 1.0 to 30 million and about 20% of amylose with average molecular weights ranging from about 40,000 to 300,000. The structure of amylopectin and amylose may be illustrated by Formulas I and II wherein n is the number of linear and branched anhydroglucose repeating units. ##STR1##